The deep level transient spectroscopy (DLTS) method is commonly known as one technique for measuring a deep impurity level of a semiconductor crystal. After the original work by D. V. Lang (J. Appl. Phys., Vol. 45, No. 7, pp. 3023-3032, 1974), the DLTS method has been improved and extended by many researchers. However, even in the improved versions it is still required to form an electrode on a specimen. DLTS instruments commercially available include ones from Hewlett Packard Co. and Bio-Rad Co., Ltd., and are based on the technique developed by D. V. Lang.
There exist chemical analysis methods, e.g., SIMS, for directly identifying atom species which form deep impurity levels. However, in these methods, a specimen is destroyed as a result of measurement.
On the other hand, it is noteworthy that the minority carrier lifetime is a parameter which is influenced by impurities forming deep levels in a semiconductor. Non-contact methods for measuring the minority carrier lifetime have already been developed, which include: R. D. Westbrook, ed., "Lifetime Factors in Silicon", ASTM, PA, 1980; and a laser/microwave method of H. Jacobs et. al., J. Appl. Phys., Vol. 30, No. 7, pp. 1054-1060, 1959. Instruments based on such methods are now marketed by several companies. Although these methods and instruments are non-destructive, they cannot determine the level of related impurities.
Since a conventional DLTS method necessarily requires the formation of an electrode on a specimen, it cannot be non-destructive or of non-contact. In the DLTS method, the quality of the electrode formation process affects measurement results as disturbances. Further, in practicing the DLTS method, it is necessary to cool a specimen by liquid nitrogen, etc., which requires a measuring instrument to have a vacuum system. As a result, it is difficult to simplify the instrument and there exist many restrictions on the specimen shape.
As described above, other methods such as an SIMS analysis are essentially destructive. Further, parameters which can be obtained from a semiconductor wafer in a non-contact manner are restricted to the shape/dimension, resistivity, minority carrier lifetime, etc.
In the present situation of the art as described above, it is pointed out that requirements for the quality of semiconductor wafers are now becoming so severe that satisfactory evaluation of semiconductor devices cannot be attained without evaluating substrates themselves which constitute the devices. Therefore, it has been desired to establish a method and apparatus capable of quantitative measurement of a deep impurity level in a non-contact, non-destructive manner.